The present invention relates to a magnetic flux detector which is placed in lines of magnetic flux to detect the magnetic flux.
Electromagnetic devices whose solenoids are supplied with excitation currents to generate magnetic fluxes are well known, and electromagnetic clutches, which utilize electromagnetic forces generated by such electromagnetic devices for engaging the clutch discs, are also well known. For example, Japanese Laid-Open Patent Publication No. H04(1992)-312217 discloses an arrangement in which a differential limiter that is activated by the engagement of a main clutch is provided to the rear differential gears of an automobile, the main clutch being operated by a pilot clutch which comprises an electromagnetic clutch. In this arrangement, the duty value for the excitation current supplied to the solenoid of the electromagnetic clutch is controlled in feedback of the engaging force (clutching force) of the electromagnetic clutch, which force is being detected. The detection of the engaging force of the clutch for this feedback control is performed by detecting a leakage flux of the main magnetic flux, which is being generated by the solenoid, with a magnetic flux density sensor comprising a Hall element.
In such an arrangement, as a leakage flux from the main magnetic flux is determined in magnetic flux density by a Hall element, in the assemblage, a little misalignment of the magnetic flux density sensor, which comprises a Hall element, affects the accuracy of the detection. This is a problem that affects accuracy in detecting the magnetic flux. In addition, if an attempt to improve the efficiency of the magnetic circuit is attempted, then such an attempt will cause a reduction in the magnitude of the leakage flux and thus will also lead to a reduction in the accuracy of the detection. In other words, although the efficiency of the magnetic circuit is improved when the leakage flux is reduced, the reducing of the leakage flux can reduce the accuracy of the magnetic flux detection.
To solve such problems, the applicant of the present invention has designed a magnetic flux detector which comprises a search coil. This search coil is provided in a path where lines of magnetic flux will pass when a magnetic flux is generated. The detector is used to measure the electromotive-force induced in the search coil by the changing magnetic flux and to integrate the electromotive force for a time period for detection of the magnetic flux. Because the search coil can detect the main magnetic flux, which passes through the path where the search coil is positioned (such a path is hereinafter referred to as a magnetic path), use of this magnetic flux detector solves the above mentioned problem of the prior-art magnetic flux detector that comprises a Hall element.
However, for a device that generates an electromagnetic force, for example, a clutch mechanism, this magnetic path, where the search coil is placed, is located inside the mechanism, so the search coil is exposed to a harsh environment which is created and affected by the operational condition of the clutch. As the ambient temperature to which the search coil is exposed changes over a time, the operational temperature of the search coil also changes appreciably, causing the internal resistance of the coil to change accordingly. If there is a change in the internal resistance, then there will be a corresponding difference in the electromotive force induced in the coil even though the magnetic flux itself experiences no change. This is another problem that prevents accurate magnetic flux detection.
It is an object of this invention to provide a magnetic flux detector which comprises a search coil provided in a magnetic path where a main magnetic flux exists. Such a detector enables a direct and highly accurate detection of the magnetic flux even though a leakage flux is minimized.
Another object of this invention is to provide a magnetic flux detector which performs a highly accurate magnetic flux detection even if the internal resistance of the search coil changes in response to fluctuations in the operational temperature of the search coil.
A magnetic flux detector according to the present invention comprises a magnetic flux generator (for example, the exciting coil 13 and the armature member 11 described in the following embodiment), a search coil, which is placed in a magnetic path where a magnetic flux generated by the magnetic flux generator passes, and a magnetic flux calculator, which has a potential setup resistor whose resistance is larger than the internal resistance of the search coil. The magnetic flux generator generates a magnetic flux, and an electromotive force is induced in the search coil in response to a change in the magnetic flux. The magnetic flux calculator determines the magnitude of the magnetic flux being generated by the magnetic flux generator by measuring the electromotive force induced in the search coil.
In the magnetic flux detector, the search coil is placed in the magnetic path of the magnetic flux (so-called main magnetic flux) generated by the magnetic flux generator. Because of this arrangement, an electromotive force is induced in the search coil when there is a change in the magnetic flux. When this electromotive force is integrated for a time period, the magnetic flux that is passing through the magnetic path is determinable accurately. This arrangement effectively eliminates a problem or errors which may otherwise occur if the sensor is misaligned as in a case of prior art where the sensor comprises a Hall element to detect a leakage flux for calculation of the magnetic flux. Also, this arrangement secures the accuracy of the magnetic flux detector because it is immune to a flux leakage reduction, which can result from an improvement in the efficiency of the magnetic circuit.
In addition, the magnetic flux detector according to the present invention comprises a potential setup resistor whose resistance is larger than the internal resistance of the search coil. This resistor is placed at the inlet of the magnetic flux calculator. This arrangement effectively minimizes the effect of fluctuations in the internal resistance of the search coil on the electromotive force induced in the search coil. The reason is that the magnitude of such an influence corresponds to the ratio of the internal resistance of the search coil to the resistance of the potential setup resistor. As the fluctuation of the electromotive force induced in the search coil is minimized against fluctuations in the internal resistance of the search coil, the precision of the detector is maintained to perform an accurate magnetic flux detection even while the internal resistance of the search coil is changing in response to the fluctuation of the temperature.
The magnetic flux calculator of the magnetic flux detector according to the present invention may comprise a high potential setup circuit, which is connected to one end of the search coil and supplies a predetermined high voltage, a low potential setup circuit, which is connected through the potential setup resistor to the other end of the search coil and supplies a predetermined low voltage, and an electrical potential measuring device, which is connected to a predetermined point between the potential setup resistor and the latter end of the search coil and measures the electrical potential at this predetermined point. In this arrangement, while the high voltage set by the high potential setup circuit and the low voltage set by the low potential setup circuit are constant, if the internal resistance of the search coil fluctuates, then the voltage measured at the predetermined point by the electrical potential measuring device is affected from the fluctuation only to a degree which corresponds to the ratio of the internal resistance of the search coil to the resistance of the potential setup resistor. Because this ratio is made comparatively small by setting the resistance of the potential setup resistor substantially larger than the internal resistance of the search coil, the error in the measurement of the electromotive force induced is minimized against changes in the internal resistance of the search coil.
It is preferable that the magnetic flux calculator be constructed to provide the high voltage, which is supplied by the high potential setup circuit, and the low voltage, which is supplied by the low potential setup circuit, in positive values. Then, the electrical potential measuring device will measure only positive values. In this case, the circuit arrangement of the measuring device can be simplified for cost-effectiveness.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.